iPhone hacker extraordinaire Erica Sadun made an application for the iPhone that will listen to music and identify it! She writes –

Download a copy of the Listen beta. It’s still *very* beta. Install it on your iPhone, get near music and run it. It will sample the music for 5 seconds and then attempt to contact the id server. If it succeeds, great, you’ll see the song, artist and album. If it fails, nothing happens. You just wait 30 seconds and if you haven’t gotten an ID, quit and restart the program.

iPhone audio recognition [via] – Link.